Thermally-induced transition in aqueous solution of cationic surfactant containing aromatic ring

Abstract

Heat capacities of aqueous solutions of cetyltrimethylammonium (CTA) salicylate, of CTA m-hydroxybenzoate and of CTA p-hydroxybenzoate were measured using a scanning microcalorimeter. Only the salicylate solution exhibited heat-capacity anomaly around 330 K, depending on the heating rate. The transition enthalpy was 3.5±0.2 kJ mol−1, which was similar to that observed in solution of 1:1 intermolecular compound between CTA bromide (CTAB) and o-iodophenol (OIPh). The enthalpy of formation ΔHf of the 1:1 intermolecular compound from CTAB and OIPh was determined by measuring the enthalpies of solution of the relevant crystals into ethanol. Positive value ΔfH=3.0±0.3 kJ mol−1 was explained from a large difference between the heat capacities of the 1:1 compound and 1:1 mixture of the component crystals.